USB3 sustained transfer rate

[teddymills1-Re5JQEeQqe8AvxtiuMwx3w at public.gmane.org]

Ubuntu 10.10
M4A88T-V EVO/USB3
Vantec USB3 case with WD 2TB green
I dont know what the speed of the WD Green is, but it is definitely not 
the fastest.
2 USB3 ports and use 1 USB3 port.

With USB2 it sustains 30MB/sec.
With USB3 the speed varies from 70MB to 105MB/sec (averaging 90MB/sec)

Any idea why it could vary from 70 to 105 ?
















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